Nozzle loads on a flat head or a flat plate

WRC 537 gives local stresses in cylinders and spheres. Its curves are read against γ = Rm/T, and a flat plate has no finite radius, so there is no curve to read. The difference is more than a missing chart. A curved shell carries pressure mainly as membrane stress. A flat plate cannot: it carries pressure by bending, and that bending grows with the square of the span.

A nozzle on a flat head therefore sits in a field that is already bending under pressure, and its own loads add plate bending on top. How much of each, and where the largest stress sits, depends on how the plate's edge is held. None of this is in the bulletin's curved-shell solution.

The parent is a flat plate, circular by its diameter or rectangular by its two side lengths, with a thickness and a corrosion allowance. Pressure acts on the inside face. The same shell elements as on a cylinder carry membrane force and bending in one layer, and the nozzle shares nodes with the plate along the junction.

• A round radial nozzle, at the centre of a circular plate or off it, inside the edge.
• Tilted nozzles, obround and tombstone openings, and up to four attachments together.
• Clips, a lug as a clip, and a trunnion that does not pierce the plate.
• An offset (hillside) nozzle is refused: on a flat plate it is only a nozzle in another place, so the model asks for the nozzle to be moved instead.

The plate's edge is given with the plate, because it is a fact of the design and not a modelling choice. Clamped holds every translation and rotation at the edge: an integral flat head welded to its shell. Simply supported holds the translations and leaves the edge free to rotate: a cover bearing on a flange or a ledge.

A flat head welded to a real shell is neither. The shell beside it turns a little with the plate, so the edge is stiffer than a simple support and softer than a clamp. Our scope notes propose an edge that models the plate welded to a short length of its shell. That is not built. Until it is, running the job with each edge shows how much the nozzle stresses depend on the assumption.

Division 2 Table 5.6 treats the pressure bending of a flat head as primary, because the plate needs it to carry the load to its edge. The model applies that away from the nozzle. At the nozzle the junction is checked as it is on a shell, read at the weld toes.

Our scope notes list flat heads with nozzles among the shapes where stress classification is a judgement. A limit-load analysis needs no classification, which is why it suits them. In our verification notes the limit load of a simply supported circular plate came out +0.00 % against its reference on 1216 elements. A clamped plate was +7.8 %, +4.2 % and +2.9 % on 1216, 4864 and 9765 elements, because the hinge forms at the rim, where the graded mesh is coarse. A clamped flat head needs a fine mesh at its rim.

The thickness of the flat head and the reinforcement of the opening in it are design-by-rule checks, in Division 1 UG-34 and UG-39. The model does not size either. It answers a different question: whether the nozzle loads, with pressure, are carried at the junction and in the plate round it.